Update JSON encoder to provide line delimited JSON

[dlindhol]

The current JsonEncoder represents each Sample (of an uncurried dataset) as a JSON array. The resulting dataset encoding is thus a sequence of JSON arrays with no delimiter. This ticket is to align this output option with line delimited JSON.

• Add a newline (\n) to each encoded sample
• Change the name of the encoder. JsonLinesEncoder?
• Change the suffix for service requests to "jsonl" (which we prefer over "ndjson" and "ldjson").
• Change the media type in the response header. Of the various flavors of line delimited JSON, the only one to specify a media type is application/x-ndjson. However, there are cases of "application/jsonl" in the wild.

[lindholc]

See #179.